Is the questioner sinful because his 13-year-old sister does not wear the hijab, knowing that she refuses to wear it under the pretext of not being ready, and his father refuses to force her to wear it?

The hijab is an obligatory duty upon the Muslim woman, and covering the entire hair of the head is a part of it. If the sister reaches puberty, she must wear the hijab and conceal herself. She should be advised kindly, and the virtues of the hijab should be explained to her, as it is pleasing to the Lord, a sign of chastity, and a protection for the woman. It is best that the advice comes from someone she respects. You do not have the right to force her, and you bear no sin if she goes out immodestly. Her father is her guardian and is responsible for her, and he will be held accountable for her on the Day of Judgment, for Allah the Almighty says: "O you who have believed, protect yourselves and your families from a Fire" [Quran 66:6], and the Prophet, peace and blessings be upon him, said: "All of you are shepherds and all of you are responsible for your flock." The father must be advised and reminded of his duty. It is permissible to abandon a disobedient person if there is hope of reforming them, otherwise, abandoning abandonment to win their heart over is preferable.